A standard utility knife has an elongated handle having an outer end and a longitudinally extending flat blade secured in the handle and projecting outwardly from the outer end. The blade has a transverse outer edge and a longitudinally extending side or front cutting edge meeting at a point. Such a knife is useful for opening packages, including cartons and bags, and is typically carried in the pocket or tool belt of the person using it.
In order to protect the user from injury by the cutting edge and point it is known from German patent 3,116,354 and U.S. Pat. No. 4,086,698 to form the handle with a longitudinally outwardly open and longitudinally extending guide slightly offset laterally from the cutting edge of the blade. A longitudinally extending pin has an outer end carrying a bumper or shield and is longitudinally displaceable in the guide between an extended positions with the outer end lying longitudinally outward of the point and a retracted position with the outer end longitudinally inward of the point. A spring is braced between the pin and the handle and urges the pin into the extended position.
Thus to use such a knife the shield end of the guard pin is pressed against the item to be cut or slit until it is pushed inward past the point which can then cut into the item. The guard does indeed protect against casual contact with the point, but is not suitable for use in opening a bag as the guard can easily get caught in the slit being cut, for instance if the knife is moved briefly backward. Furthermore when used on particularly thick bag materials, such as thick plastic sheeting or fabric of multiple layers, it is quite hard to use.
German utility model 1,899,717 describes a system where a protective cap is urged outward past the blade and can be pressed inward against spring force to let the blade project through a slit in the cap for a combined stabbing/cutting operation as in opening a bag. A similar pivotal U-shaped cap is also described in German patent document 1,121,972. These systems are particularly unsuitable for opening bags and the caps do not move reliably and the knife must be brought at a specific angle into engagement with the bag.
In U.S. Pat. No. 5,325,594 of Szanfranski a knife has a pivotal U-shaped guard that serves for protecting the blade and compressing the item being slit. A catch is operated to retract the guard and, once the catch is operated, the blade is left sticking out and is not protected at all.
The system of U.S. Pat. No. 4,393,587 of Kloosterman has a blade mounted on a carrier so it can be pushed against a spring force through an end guard. The guard at the end is configured to make it hard for the blade to poke through a bag and then slide along it.
In U.S. Pat. No. 2,743,523 there is a double bow-formation to each side of the outer region of a square blade. This double guard assembly is made of elastic material so it must be deformed as the knife is pulled along something to be cut. Such a knife is almost impossible to use in a stabbing/cutting operation.
Finally, U.S. Pat. No. 4,569,133 of Schmidt had a blade fixed in the handle but projecting through a shield whose longitudinal position can be set to establish an exact depth of cut. This guard is not normally movable and normally the blade always projects outward past it, so it does not really allow the tool to be pocketed when not in use.